The valve face of an air intake/exhaust valve of an internal combustion engine is required to have a high wear resistance and a hot corrosion resistance, since it comes into contact with a valve seat to open/close the combustion chamber of the engine. Thus, there have been made many proposals to improve wear resistance and hot corrosion resistance of valves, involving provision of appropriate forging temperatures and a forging rate. However, any of the past proposals fails to provide the valve face with sufficient hardness and fails to prevent deposition of combustion residues and generation of impressions in the valve face, which lowers a blow-through characteristic of the valve face. This is the case particularly with valves for diesel engines that use a low grade fuel.
A patent document 1 listed below discloses a measure for solving this problem.
This prior art forging method utilizes a Ni-type precipitation hardened alloy as the valve face material, which is forged in the temperature range from 20 to 500° C. so as to create radial slip deformations in the valve face.
Consequently, the valve face is hardened to an extent that the valve face is scarcely impressed with combustion residues and wear resistance is greatly improved. In addition, this method can provide the valve with harder deep layers near the periphery of the valve than shallow layers inside the periphery, thereby greatly prolonging the life of the valve.